The invention relates to methods and apparatus for milling openings in downhole structures in a wellbore.
To produce hydrocarbons from an underground formation or to inject fluids into an underground formation, wellbores are drilled through the earth subsurface to the desired formation. Such wellbores may be vertical, deviated, or horizontal wellbores. Wells may also be multilateral wells, which have multiple lateral branches that extend from a parent wellbore (also referred to as the main bore).
After a wellbore has been drilled into the earth subsurface, it is typically lined with casing or another type of liner. Casing extends from the well surface some distance into the wellbore. In some wells, liners are also used to line other portions of a wellbore.
In some cases, it may be desirable to change the trajectory of a wellbore after the wellbore has been drilled and the casing or liner has been cemented in the wellbore. The change in trajectory may be desired to reach better producing zones of a formation. Further, lateral branches may be extended from a cased or lined main bore to provide a multilateral well.
To change the trajectory of the wellbore or to add a lateral branch, windows are formed in the casing or liner to enable drilling of the lateral bore. The casing or liner window is generally cut by a milling assembly having one or more mills. The peripheral surfaces of the mills are generally covered with abrasive or cutting inserts made of a hard material, such as sintered tungsten carbide compounds braised on a steel mandrel. The mills are designed to cut through a steel casing or liner. A whipstock is generally set in the wellbore before the milling assembly is run into the wellbore. The whipstock is located in the proximity of the region in which the lateral bore is to begin. The whipstock provides a slanted surface that guides the mills of the milling assembly into the adjacent casing or liner. The whipstock pushes the milling assembly towards the casing or liner wall under action of a downward force on the milling assembly.
Although a whipstock is expected to support some milling damage, it may be difficult to predict how much whipstock material is left after milling has been performed. In addition, after milling operations have been completed, it may be difficult to retrieve the damaged whipstock, which can lead to a major obstruction of the well and subsequent abandonment of the section of the well below the whipstock. In addition, conventional milling assemblies may not provide adequate control of the window geometry.
In general, improved method and apparatus are provided for milling windows or other openings in well casings or liners or other downhole structures. For example, a milling apparatus to mill a window through a downhole structure having a longitudinal axis includes a deflector having a reaction surface, and a motion mechanism adapted to move the deflector generally along the longitudinal axis. A mill is adapted to be engaged with the reaction surface and to move generally along the longitudinal axis with the deflector.
Other features and embodiments will become apparent from the following description, from the drawings, and from the claims.